1. Field of the Invention
The present invention relates to a solid-state image capturing device comprising semiconductor elements for performing a photoelectric conversion on and capturing an image light from an object, a method for manufacturing the solid-state image capturing element, and an electronic information device, such as a digital camera (e.g., digital video camera and digital still camera), an image input camera, a scanner device, a digital copying machine, a facsimile machine and a camera-equipped cell phone device, having the solid-state image capturing device as an image input device used in an image capturing section of the electronic information device.
2. Description of the Related Art
The conventional solid-state image capturing device described above includes a CCD (Charge Coupled Device, electric charge coupling element) image sensor (referred to as CCD, hereinafter), and a CMOS (Complementary Metal Oxide Semiconductor) image sensor.
Such a solid-state image capturing device is used for various applications including a digital camera, a video camera, a camera-equipped cell phone device, a scanner device, a digital copying machine, and a facsimile machine. Further, as products using such a conventional solid-state image capturing device spread, the demand is increasing for decreasing the size and lowering the price of the solid-state image capturing device, in addition to higher functionality and higher performance of the increased number of pixels and increase on the light receiving sensitivity.
As such, when advancement for the solid-state image capturing device includes downsizing and a high pixel rate, while lowering of the price is requested, the pixel size is also reduced. Along with such a reduction of the pixel size, the light receiving sensitivity, one of the basic performances of the conventional solid-state image capturing device, is decreased, making it difficult to capture a clear image in a low luminous intensity environment. Therefore, it is a significant problem to be solved as to how the light receiving sensitivity can be improved per unit pixel.
Accordingly, Reference 1, for example, discloses a method for forming a microlens with an organic high molecule material on a color filter, as a method for improving the light receiving sensitivity of the solid-state image capturing device. In addition, References 2 and 3 disclose, in addition to a method for forming a microlens on a color filter, a method for forming what they call an inner-layer lens inside a laminated structure located below the color filter and in between a light receiving section and the color filter. According to Reference 2, an upwards convex shaped inner-layer lens is formed by transfer printing, whereas, according to Reference 3, a downwards convex shaped inner-layer lens is formed using a concave shaped well between a photoelectric conversion section and a transfer electrode. Herein, Reference 2 where an upwards convex shaped inner-layer lens is formed by transfer printing, will be described with reference to FIGS. 8 and 9(a) to 9(e).
FIG. 8 is a longitudinal cross sectional view schematically illustrating an exemplary essential structure of one pixel of a conventional CCD type solid-state image capturing device disclosed in Reference 2.
As illustrated in FIG. 8, a conventional CCD type solid-state image capturing device 20 includes impurity diffusion layers, such as a photoelectric conversion section 22 (light receiving section), a readout gate section 23, a CCD transfer channel 24, and a channel stopper 25, that constitutes each pixel, the impurity diffusion layers formed in a semiconductor substrate 21 under the front surface side thereof.
A transfer electrode 27 is formed above the readout gate section 23 and the CCD transfer channel 24 with an insulation film 26 interposed therebetween. The transfer electrode 27 has a function to read out a signal charge from the photoelectric conversion section 22 (light receiving section) to the CCD transfer channel 24, besides a function to transfer an electric charge in a predetermined direction. A light shielding film 29 is formed above the transfer electrode 27 with an inner-layer insulation film 28 interposed therebetween. The light shielding film 29 has an opening above the photoelectric conversion section 22 (light receiving section).
A first planarization film 30 formed of BPSG (Boro-Phospho-Silicate Glass) and the like is formed on the insulation film 26 and the light shielding film 29, and an inner-layer lens 31 formed of a silicon nitride film and the like is formed thereon. The inner-layer lens 31 is formed in such a manner to be disposed above the photoelectric conversion section 22. A second planarization film 32 is formed on the first planarization film 30 and the inner-layer lens 31, and its surface is planarized. On the second planarization film 32, a color filter 33, in which three primary colors of red, green and blue (R, G and B) are combined and arranged for every pixel, is formed. Further, a microlens 35 is formed above the color filter 33 with a protection film 34 interposed therebetween and in such a manner to be disposed above the photoelectric conversion section 22.
Herein, a method for manufacturing the conventional CCD type solid-state image capturing device 20 will be described in detail with reference to FIGS. 9(a) to 9(e).
FIGS. 9(a) to 9(e) are respectively longitudinal cross sectional view of essential parts for illustrating each manufacturing step up to the inner-layer lens forming step of the conventional CCD type solid-state image capturing device illustrated in FIG. 8.
First, as illustrated in FIG. 9(a) and as an impurity diffusion layer forming step, a predetermined impurity ion implantation is performed into the semiconductor substrate 21, and the photoelectric conversion section 22, readout gate section 23, CCD transfer channel (transfer section) 24, and channel stopper 25, are respectively formed.
Subsequently, the insulation film 26 is formed on the surface of the semiconductor substrate 21, and a film thickness of 300 nm, for example, of a transfer electrode material is layered thereon to form a predetermined pattern of the transfer electrode 27. The transfer electrode 27 is covered by the inner-layer insulation film 28, and a film thickness of 200 nm, for example, of the light shielding film 29 is formed in such a manner to have an opening above the photoelectric conversion section 22, with the inner-layer insulation film 28 interposed therebetween.
Next, as illustrated in FIG. 9(b) and as a first planarization step, a BPSG film, for example, is accumulated at the film thickness of about 600 nm on the light shielding film 29 and the insulation film 26 by the atmospheric pressure CVD method, the BPSG film being set at a predetermined phosphor concentration and a boron concentration, and the BPSG film is reflown under a high temperature of more than 900 degrees Celsius. As a result, the first planarization film 30 is formed.
Next, as illustrated in FIG. 9(c) and as an inner-layer lens forming step (first step: silicon nitride film filing step), a silicon nitride film 36 is layered at the film thickness of, for example, about 1200 nm by a plasma CVD method on the first planarization film 30.
Further, as illustrated in FIG. 9(d) and as the inner-layer lens forming step (second step: resist lens pattern forming step), a positive type resist is applied on the silicon nitride film 36 and a patterning is performed with the same lens shape in order to obtain a desired inner-layer lens 31. Subsequently, reflow is performed at more or less 160 degrees Celsius, for example, to form a resist pattern 37 having the lens shape on the silicon nitride film 36.
Subsequently, as illustrated in FIG. 9(e) and as the inner-layer lens forming step (third step: transferring step), dry etching is performed under a condition with strong anisotropy to transfer the lens shape of the resist pattern 37 into the silicon nitride film 36 in order to form the inner-layer lens 31.
Further, in order to improve the light focusing rate of the inner-layer lens 31, the second planarization film 32, which is formed of a material with a low refractive index, is formed in such a manner to cover the inner-layer lens 31 and the surface is planarized, as illustrated in FIG. 8. Subsequently, the color film 33, the protection film 34, and the microlens 35 are respectively formed to manufacture the conventional solid-state image capturing element 20 described above.
The layering of the silicon nitride film 36 has been described above; however, without the limitation to this, spattering can be performed under the condition of Ar: 100 sccm, O2: 10 sccm as the gas type and its flow rate, with (Pb, La) (Zr, Ti) O3 as a target so as to layer a PLZT film when the microlens 35 and the inner-layer lens 31 are formed using the refractive index variable material described above.
On the other hand, Reference 4 discloses a refractive index variable microlens that utilizes a characteristic to change the refractive index by applying a voltage (the Pockels effect) as another method for improving the light receiving sensitivity of a solid-state image capturing element.
When such a solid-state image capturing element is installed in a video camera and the like, the F value of the lens provided on the camera side is changed so as to obtain an appropriate exposure in accordance with image capturing conditions. Therefore, the light entering the solid-state image capturing element through the lens of the video camera changes its angle by the diaphragm of the video camera, and an oblique light enters as well as a parallel light.
Due to this, electrodes are provided at the top and bottom sections of the microlens and the inner-layer lens in order to constantly receive an incident light at the photoelectric conversion section in response to the diaphragm (the amount the lens is opened or closed) of the lens of the video camera. By applying a voltage to the microlens and the inner-layer lens, the refractive index of the microlens and the inner-layer lens can be arbitrarily changed.
In this case, a refractive index variable material layer (e.g., PLZT, LiNbO3) formed of an electro-optics ceramics is used as the material for the microlens or the inner-layer lens. PLTZ is a piezoelectric material where a part of Pb is replaced with La in a lead zirconate titanate solid solution (PbTiO3.PbZrO3).    Reference 1: Japanese Patent No. 2945440    Reference 2: Japanese Laid-Open Publication No. 11-40787 (Japanese Patent No. 3809708)    Reference 3: Japanese Laid-Open Publication No. 11-87672    Reference 4: Japanese Laid-Open Publication No. 2001-60678